1. Field of the Invention
This invention generally relates to an ion implantation process. Certain embodiments relate to monitoring and/or evaluating an ion implantation process by measuring the optical properties of a wafer during processing.
2. Description of the Related Art
Ion implantation is typically used to introduce impurity materials, or dopant ions, into the surface of a semiconductor device. Because ion implantation offers several advantages over diffusion doping, it is becoming an integral part of many semiconductor fabrication processes. Ion implanters, however, are among the most sophisticated and complex systems in semiconductor manufacturing. In order to be utilized efficiently, ion implanters may require frequent monitoring and careful operation. For example, ion implantation systems may introduce a number of defects (e.g., non-uniformities) into the semiconductor process. Such defects may cause significant yield problems. A defect may result from contamination, such as material that is sputtered from the semiconductor substrate or wafer surface as a result of ion bombardment. The accumulation of contaminants over time may adversely affect the performance of the ion implanter and may reduce the wafer yield below acceptable levels.
In order to take advantage of the benefits of ion implantation processes, extensive characterization is typically performed to ensure that the process is within design tolerance. Ideally, extensive characterization of the process takes place both during process development and during process control of manufacturing processes. Typically, ion implantation processes are characterized by implant dose, uniformity of implant dose across the wafer, uniformity of dose across several wafers, and implantation depth profiles. Accurate measurement of the implant dose, however, may be a difficult task because the measurement is generally based on integrating the beam current. Error sources may be introduced into the measurement of the integrated beam current by interactions between the beam and electrons, neutrals, and negative ions as well as secondary particles which may be emitted as a result of ion bombardment of the target.
One process control method that may be used to monitor and assess an ion implantation process involves determining the sheet resistance of implanted wafers using a four-point probe technique. The four-point probe technique involves using a colinear probe arrangement which is arranged to contact the implanted regions on the semiconductor wafer. In operation, a current is passed between the two outer probes and the voltage drop across the two inner probes is measured. The test is typically performed twice in order to eliminate thermoelectric heating and cooling errors from the measurements. The first test involves passing a current in a first direction, referred to as the forward direction. The second test then involves passing the current in a second direction, opposite to the first direction, referred to as the reverse direction. The two voltage readings may then be averaged. The test may also be performed at several different current levels because testing at an improper current may cause the forward and reverse test results to differ or to cause the readings to drift.
Because the impurity regions must be electrically activated prior to electrical testing, this process control method may introduce several additional processing steps to the fabrication of a monitor or test wafer. For example, the impurity regions are typically electrically activated by rapid thermal processing. During this processing, masking materials such as photoresist may volatilize or reflow which may cause contamination or removal problems in subsequent processing. Therefore, the photoresist or other masking material is typically removed prior to electrically activating the impurity regions. Consequently, the time required to perform these additional processing steps increases the processing time and cost associated with electrical testing. Furthermore, long test times may be extremely costly if additional wafers have been processed incorrectly before the electrical test results were available. Suspending processing until the electrical test results are available, however, may also be costly due to production delays and idle production tools.
Optical dosimetry may also be used to monitor and control ion implantation processes. This technique measures the darkening, or increased optical absorption, of photoresist that occurs due to exposure to ion beams. Monitor wafers may be prepared by coating photoresist onto a transparent substrate. The monitor wafer is then scanned with a dosimeter to determine its background optical absorption. The wafer may then be subjected to an ion implantation process. After implantation, the wafer may be scanned again using the dosimeter, and the background optical absorption may be subtracted from this data. In this manner, the distribution of implanted ions across the entire wafer may be measured and plotted on a contour plot. From the plot of this data, variations and trends in the distribution of implanted ions across the wafer may be discerned. Additionally, the extent of the variations and trends may be analyzed to determine if the ion implantation process is within design tolerances.
Optical dosimetry may be particularly useful when making qualitative assessments of the performance of an ion implantation process. In addition, this technique allows the diagnosis of an ion implantation step to be done with greater resolution and sensitivity. For example, scan lock-up, non-linear scanning, and loss of beam diameter control may be detected using the optical dosimetry technique. Scan lock-up is a common problem which may result from overlap of individual Gaussian beam traces and may cause dopant non-uniformities across the wafer. Scan lock-up may be particularly problematic at low dopant doses. For example, low dopant doses typically have decreased beam currents and scan times which may lead to significant overlap in some regions of the wafer and doping level gaps in other regions of the wafer. Non-linear scanning may result from beam neutralization caused by collisions between ions and residual gas atoms in the beam chamber and neutralization from thermal electrons caught in the beam. Positive charge build-up on an insulating layer on the wafer may also result in non-linear scanning because the build-up may alter the charge balance in the ion beam and lead to significant dose variations across the wafer. Loss of beam diameter control, or defocusing, is another common problem in ion implantation which may result from separation of the beam due to repulsion of like charges. Defocusing of the beam may also cause uneven ion density and non-uniform implant concentrations in the wafer.
There are several disadvantages, however, in using the optical dosimetry technique to monitor ion implantation processes. For example, because only a photoresist-coated transparent substrate may be used in this technique, additional processing steps and materials are typically required. Furthermore, the testing method may not accurately predict the ion implantation performance of a product wafer which may have a topography which differs dramatically from a substrate coated with a planar resist layer. For example, positive charge build-up on a wafer may be particularly problematic when implanting into an insulating layer, such as resist or silicon dioxide, which may lead to significant dose variations across the wafer. The topography or patterning of the masking layer on a product wafer may cause additional localized positive charge build-up. A wafer having a planar resist layer may not accurately show the localization of positive charge build-up. Therefore, using a dissimilar test wafer may not accurately detect all of the potential problems that may occur in an ion implantation process.
Another process control method which may be used to monitor and control ion implantation processes involves the use of modulated and non-modulated reflectance. An example of such a process control method and apparatus is illustrated in U.S. Pat. No. 5,074,669 to Opsal which is incorporated by reference as if fully set forth herein. A modulated signal may be obtained by periodically exciting a semiconductor substrate with a focused, intensity modulated, pump laser beam. The reflected power of the probe is measured to yield a first modulated reflectance signal. A second measurement is taken of the non-modulated reflectance signal using the argon laser in a non-modulated operating configuration. In addition, a third measurement is also taken of a non-modulated reflectance signal using a second laser beam in a non-modulated operating configuration. These three measurements may be correlated with the dosage level. The three reflectance measurements, therefore, provide three independent measurements. This data may then be used in conjunction with a mathematical model to characterize an ion implantation process.
There are several disadvantages, however, to using the modulated and non-modulated reflectance technique as a process control method for ion implantation. For example, certain films will exhibit a great sensitivity, or signal response, to certain wavelengths. In the above example, therefore, if a masking layer is chosen that is substantially different than silicon dioxide, it may be necessary to change the lasers that are used in the system. Measurements at single wavelengths may also inherently have more ambiguities than multiple wavelength measurements. Therefore, this approach may not be particularly useful when the index of refraction of the material is not accurately known. Furthermore, the number of layers which may be analyzed using this method is proportional to the number of independent measurements that are taken. Analysis of complex multi-layer stacks, therefore, would necessitate performing additional measurements. Using single-wavelength sources and analyzing multi-layer stacks may require incorporating additional lasers into the system.
Alternative nondestructive optical testing, such as spectroscopic ellipsometry and spectroscopic reflectometry, is becoming increasingly popular to characterize semiconductor films. In these techniques, electromagnetic radiation may be impinged upon a sample, and the reflected radiation may be measured. Reflectance data may then be used to determine characteristics of the semiconductor films such as the thickness of a single or multiple layer or the refractive index. Many different materials may be characterized using reflectance data including, for example, photoresist, silicon oxide, silicon nitride, titanium nitride and polysilicon. Spectroscopic ellipsometry involves impinging an incident radiation beam having a known polarization state on the sample and measuring the polarization of the reflected radiation. Spectroscopic reflectometry involves impinging an incident radiation beam on the sample and measuring the intensity of the reflected radiation. The incident radiation, in both techniques, may include multiple frequency components which provide reflectance data for at least two frequency components. Examples of spectroscopic reflectometers and ellipsometers are illustrated in U.S. Pat. No. 4,999,014 to Gold et al., U.S. Pat. No. 5,042,951 to Gold et al., U.S. Pat. No. 5,412,473 to Rosencwaig et al., U.S. Pat. No. 5,581,350 to Chen et al., U.S. Pat. No. 5,596,406 to Rosencwaig et al., U.S. Pat. No. 5,596,411 to Fanton et al., U.S. Pat. No. 5,747,813 to Norton et al., 5,771,094 to Carter et al., U.S. Pat. No. 5,798,837 to Aspnes et al., U.S. Pat. No. 5,877,859 to Aspnes et al., U.S. Pat. No. 5,889,593 to Bareket et al., U.S. Pat. No. 5,900,939 to Aspnes et al., U.S. Pat. No. 5,917,594 to Norton and U.S. Pat. No. 5,973,787 to Aspnes et al., all of which are incorporated by reference as if fully set forth herein. Additional examples of spectroscopic devices are illustrated in PCT Application No. WO 99/02970 to Rosencwaig et al. and is incorporated by reference as if fully set forth herein.
Recent advances in spectroscopic ellipsometers and spectrophotometers have provided an incident radiation beam having a reduced spot size. As such, a very small region of the semiconductor film may be analyzed using these techniques. In this respect, a single feature of a semiconductor device, such as a masking layer over the gate conductor of a single transistor, may be characterized using these methods. Examples of focused beam spectroscopic ellipsometry and reflectometry methods and systems are illustrated in U.S. Pat. No. 5,608,526 to Piwonka-Corle et al., U.S. Pat. No. 5,859,424 to Norton et al., and U.S. Pat. No. 5,910,842 to Piwonka-Corle et al., all of which are incorporated by reference as if fully set forth herein.
Accordingly, it would be advantageous to develop a nondestructive optical testing method to rapidly and accurately measure, assess, and monitor an ion implantation process without sacrificing product wafers or processing additional monitor wafers.
An embodiment of the invention relates to a method to evaluate an ion implantation process. A masking material may be formed on a semiconductor substrate. Any material that is substantially transparent to a portion of the light produced by an optical inspection device may be used as a masking material. In addition, the masking material may substantially inhibit implantation of dopant ions into the underlying semiconductor substrate. Alternatively, the dopant ions may be implanted into a semiconductor substrate through the masking material. Implanting the dopant ions through a masking material may enhance the distribution profile of the implanted region by randomizing the directional paths of the ions which are being driven into the semiconductor substrate. Appropriate masking materials may include, but are not limited to, a resist material, silicon dioxide, silicon nitride, titanium nitride, or polycrystalline silicon. A masking material may also include several layers of different materials such as a resist material disposed upon an inorganic material. An appropriate masking material may be determined by the semiconductor device feature which may be formed by an ion implantation process. As such, an appropriate masking material may also be determined by the ion implantation process conditions being used to fabricate the semiconductor device such as dopant species or implant energy.
The implantation of ions into the masking material may cause physical damage and chemical changes in the masking material. For example, an implanted masking material may include an upper portion which includes a physically damaged layer of the masking material. The ions which have been implanted into the masking material may be substantially disposed in a middle portion of the masking material. A lower portion of the masking material may be substantially free of implanted ions. Therefore, the implantation of ions into the masking material may also alter an optical property of the masking material. An optical property of the masking material may be measured using a broadband radiation technique. Broadband radiation techniques which may be used to measure the optical property include, but are not limited to, spectroscopic ellipsometry and spectroscopic reflectometry. Optical properties of the masking material may include a thickness of a portion of the masking material, a thickness of the entire masking material, an index of refraction, or an extinction coefficient.
In an embodiment, a characteristic of the implanted ions in the masking material may be determined. The characteristic may be a function of the measured optical property of the implanted masking material. Characteristics of the implanted ions in the masking material which may be determined include, but are not limited to, an implantation energy when the ions are implanted into the masking material, a species of the implanted ions, and a concentration of the implanted ions. The presence of implanted ions in the masking material may also be determined using the measured optical property of the implanted masking material. In an embodiment, the measured optical property of the masking material may also be used to determine a characteristic of an implanted portion of a semiconductor substrate. A characteristic of an implanted portion of a semiconductor substrate may also be a function of the measured optical property of the masking material. Characteristics of the implanted ions in a portion of the semiconductor substrate include, but are not limited to, an implantation energy and a dose of the implanted ions when the ions are implanted into a portion of the semiconductor substrate. Additional characteristics which may be determined using a measured optical property of the masking material may include a concentration, a presence, a depth, and a distribution of the implanted ions in the portion of the semiconductor substrate.
In an additional embodiment, ions may also be implanted into a portion of a semiconductor substrate. Implantation of ions into a semiconductor substrate may be performed by implanting ions through a masking material. Alternatively, at least a portion of a masking material may be removed to expose a portion of a semiconductor substrate. A semiconductor substrate may also experience significant damage due to the implantation of ions into regions of the semiconductor substrate. For example, damage to the silicon by the implanted ions may produce an amorphous layer below an upper crystalline damaged layer. As such, implantation of ions into a portion of a semiconductor may alter an optical property of the semiconductor substrate. Therefore, an optical property of an implanted portion of a semiconductor substrate may be measured and used to determine a characteristic of implanted ions in a portion of a semiconductor substrate. The characteristic of the implanted ions in a portion of a semiconductor substrate may, therefore, be a function of the measured optical property of the implanted portion of the semiconductor substrate. Optical properties of the implanted portion of the semiconductor substrates may include a thickness, an index of refraction, or an extinction coefficient. The characteristic may include any of the characteristics which are described in an above embodiment. In an embodiment, a method to fabricate a semiconductor device may include implanting ions into a wafer, measuring an optical property of the wafer, and determining at least one characteristic of the implanted ions in the wafer. The wafer may include a masking material arranged upon a semiconductor substrate.
In an embodiment, a reference wafer may be formed by implanting ions into a first masking material which may be formed on at least a portion of a first semiconductor substrate using first ion implantation conditions. A product wafer may also be formed by implanting ions into a second masking material which may be arranged over at least a portion of a second semiconductor substrate using second ion implantation conditions. The first and second masking materials formed on the semiconductor substrates may be substantially identical. In addition, the first and second semiconductor substrates may also be substantially identical. The first and second ion implantation conditions may include, but are not limited to, process conditions such as dopant species, a dopant dose, an ion energy, and angle of implantation, and/or a temperature of the implantation process. Parameters of an instrument used to produce the first ion implantation conditions may be substantially similar to parameters of the instrument used to produce the second ion implantation conditions. Alternatively, at least one parameter of an instrument used to produce the second ion implantation conditions may be substantially different than a parameter of an instrument used to produce the first ion implantation conditions.
An optical property of the first and second implanted masking materials may be measured using a broadband radiation technique. The measured optical properties of the first and second masking material may also be compared. As such, comparison of the optical properties of the first and second masking materials may be used to determine if the first and second ion implantation conditions produced by parameters of the instrument are substantially the same. In this manner, a quantitative relationship which describes a relationship between the measured optical properties of the first and second masking materials and the parameters of the instruments used to produce the first and second ion implantation conditions may also be determined. In an additional embodiment, ions may also be implanted into at least a portion of the first and second semiconductor substrates. Therefore, an optical property of the first and second implanted masking materials may also be measured using a broadband radiation technique. Comparison of the optical properties of the first and second semiconductor substrates may also be used to determine if the first and second ion implantation conditions produced by the parameters of the instrument are substantially similar. Furthermore, comparison of the optical properties of the first and second implanted portions of the semiconductor substrates may be used to determine a quantitative relationship between the optical properties of the implanted semiconductor substrate and the ion implantation conditions.
In a further embodiment, a computer-controlled method may be used to control an optical inspection, or spectroscopic, device. The optical inspection device may be used to measure an optical property of an ion implanted masking material. The computer-controlled method may include measuring the optical property by using an optical model to calculate the optical property of the masking material. As such, the computer-controlled method may also be used to determine an appropriate optical model to calculate the optical property. Appropriate optical models may include, but are not limited to, a cauchy model, a harmonic oscillator model, and a polynomial series expansion model. The computer-controlled method may also include determining at least one characteristic of the implanted ions in the masking material. In an additional embodiment, the method may include generating a set of data. The set of data may include measured optical properties of a masking material and determined characteristics of implanted ions in the masking material. The set of data may also include data which may be generated by using different devices which may be configured to measure optical properties and to determined a characteristic of the implanted ions. As such, the set of data may be used to calibrate or monitor the performance of additional devices. In an additional embodiment, a system may include an optical inspection device, a controller computer coupled to the device, and controller software executable on the controller computer. The controller software may be operable to implement the computer-controlled method to control the optical inspection device. In an embodiment, the computer-controlled method may be implemented by program instructions which may be computer-executable and may be incorporated into a carrier medium.
In an additional embodiment, a system may include an ion implanter and a spectroscopic device. The ion implanter may be used to produce and direct ions toward a wafer. The wafer may include a masking material arranged upon a semiconductor substrate. The spectroscopic device may be coupled to the ion implanter and may be used to measure an optical property of an implanted masking material or an implanted semiconductor substrate. The system may also be used to measure variations in an optical property of the masking material or the semiconductor substrate during implantation of ions into the wafer. Therefore, the system may also be used to generate a signature which may characterize the implantation of ions into the wafer. The signature may also include at least one singularity which may be representative of an end of implantation. As such, the system may be used to determine an endpoint of the implantation process.
In a further embodiment, a system may include a spectroscopic device, an operating system, and a dual-beam device. The spectroscopic device may be configured to measure an optical property of a wafer. The wafer may include a masking material arranged on a semiconductor substrate. In addition, the operating system may be coupled to the spectroscopic device and the dual-beam device and may be used to determine a characteristic of the implanted ions in the wafer. The operating system may also include a controller computer and controller software executable on the controller computer. The controller software may also be operable to implement a method to control the spectroscopic device and to control the dual-beam device. Additionally, the dual-beam device may be used to directly measure a thickness of the masking material arranged on the semiconductor substrate.